Signal waveform deterioration occurs when wavelength dispersion compensation of optical signals is insufficient in a high density WDM optical transmission system having a density of 10 gigabytes or more per second accompanying huge increases in communication capabilities. As a result, properly receiving such optical signals is becoming more difficult. Wavelength dispersion is the phenomenon of the spread of optical signal pulse widths depending upon the optical propagation characteristics and the shape and the like of optical fiber when transmitting optical signals having multiple wavelength bands over a long distance in one optical fiber. To reduce the deterioration of signal waveforms due to this wavelength dispersion, it is desirable to accurately conduct wavelength dispersion compensation in all the wavelength bands of the WDM light at the receiving side of the WDM optical transmission device.
For example, wavelength dispersion compensation is conventionally conducted using Dispersion Compensating Fiber (DCF) in WDM optical transmission systems of 10 gigabytes or more per second. DCF is generally used as a dispersion compensation device having fixed wavelength dispersion values. However, the wavelength dispersion of optical signals varies according to the types and lengths of the optical fiber used in the transmission lines, and according to temperature or other environmental changes. As a result, recently, methods to control dispersion compensation amounts according to environmental changes by equipping optical transmission devices with variable dispersion compensators that can precisely adjust wavelength dispersion values, have become mainstream in wavelength dispersion compensation.
For example, Japanese Laid-open Patent Publication No. 2010-141683 discloses a technique that not only reduces the waveform deterioration due to wavelength dispersion, but also reduces the effects of waveform deterioration due to external factors besides wavelength dispersion such as optical fiber deformation and the like, by measuring round-trip propagation times between receiving and transmitting side devices to derive a reference dispersion compensation value ahead of time. The reference dispersion compensation value is then used in the control of a variable dispersion compensator. Moreover, Japanese Laid-open Patent Publication No. 2008-236480 discloses a technique that includes temporarily accumulating information relating to current wavelength dispersion compensation conditions in a memory, comparing past conditions outputted from the memory with the current conditions, and then controlling dispersion amounts of a dispersion compensation device according to the results of the comparison. Furthermore, Japanese Laid-open Patent Publication No. 2007-259281 discloses a technique to optimize dispersion amounts of a variable dispersion compensator to minimize bit error correction counts measured on the receiving side.